It has been shown that an electro-optic element having a plurality of individually addressable electrodes can be used as a multigate light valve for line printing. See, for example, a copending and commonly assigned United States Patent Application of R. A. Sprague et al, which was filed June 21, 1979 under Ser. No. 40,607 on a "TIR Electro-Optic Modular With Individually Addressed Electrodes". Also see "Light Gates Give Data Recorder Improved Hardcopy Resolution", Electronic Design, July 19, 1979, Pages 31-32; "Polarizing Filters Plot Analog Waveforms", Machine Design, Volume 51, Number 17, July 26, 1979, Page 62; and "Data Recorder Eliminates Problem of Linearity", Design News, Feb. 4, 1980, Pages 56-57.
The same inventor in a copending and commonly assigned United States Patent Application of R. A. Sprague, Ser. No. D79006 on "An Electro-Optic Line Printer" disclosed the support optics and electronics for incorporating a multi-gate light valve into a line printer. The present invention is an extension of the invention set forth in Ser. No. D79006 and the specification and drawings thereof are herein incorporated by reference.
In copending application Ser. No. D79006 it is disclosed that almost any optically transparent electro-optic material can be used as the electro-optic element of such a light valve as disclosed herein. Presently, the most promising materials appear to be LiNbO.sub.3 and LiTaO.sub.3, but there are other materials which qualify for consideration, including BSN, KDP, Ba.sub.2 NaNb.sub.5 O.sub.15 and PLZT. In any event, the electrodes of such a light valve are intimately coupled to the electro-optic element and are distributed in non-overlapping relationship widthwise of the electro-optic element (i.e., orthogonally relative to its optical axis,) typically on equidistantly separated centers so that there is a generally uniform interelectrode gap spacing.
To perform line printing with a multi-gate light value of the foregoing type, a photosensitive recording medium such as a xerographic photoreceptor is exposed in an image configuration as it advances in a cross line direction (i.e., a line pitch direction) relative to the light valve. More particularly, to carry out the exposure process, a sheet like collimated light beam is transmitted through the electro-optic element of the light valve, either along its optical axis for straight through transmission or at a slight angle relative to that axis for total internal reflection (TIR). Furthermore, successive sets of digital bits or analog signals samples (hereinafter collectively referred to as "data samples"), which represent respective collections of picture elements or pixels for successive lines of the image, are sequentially applied to the electrodes. As a result, localized electric bulk or fringe fields are created within the electro-optic element in the immediate vicinity of any electrodes to which non-reference level data samples are applied. These fields, in turn, cause localized variations in the refractive index of the electro-optic element within an interaction region (that is , a light beam illuminated region of the electro-optic element which is subject to being penetrated by the electric fields). Thus, the phase front or polarization of the light beam is modulated in accordance with the data samples applied to the electrodes as the light beam passes through the interaction region.
In copending application Ser. No. D79004, herein incorporated by reference, is disclosed a thin light modulator for use in such an electro-optic line printer. According to the invention set forth therein the light valve utilizes a thin electro-optic film on a substrate, with the index of refraction of the film being higher than the substrate so that the light can be guided in it. A laser beam is caused to be guided in the film through one of the many ways noted in the literature. This beam is expanded sideways either before or after coupling into the film so that a sheet of collimated light is provided. This light is guided beneath an array of electrodes identical to the array described in copending application Ser. No. D79006, as herein incorporated by reference, which consists of an extended set of individually addressed metal fingers. As set forth in said copending applications, the light diffracted by the electrode set is reimaged onto the recording medium, with the zero order diffracted light blocked out, so that each electrode acts as a light modulator for one picture element on the output. Alternatively, the zero order is reimaged and diffracted light blocked out. By applying each bit of a serial stream of data to each electrode after serial to parallel conversion, a line recording is achieved.
The present invention, on the othr hand, is an alternative embodiment of the total internal reflection light modulator/scanners described in the above identified copending patent applications. In these patent applications, a line of light was modulated spatially by an array of electrodes placed near or on an electro-optic crystal, with an individual driver attached to each individually addressable electrode, to achieve recording of digital data on a light sensitive material. In these other disclosures, the light was totally internally reflected from the active surface, achieving interaction with fringing electric fields as it passed near the surface of the material at close to grazing incidence. According to the present invention, the electro-optic crystal is polished to form a thin sheet of material, with the light focused into the material in two different ways to achieve interaction with the electric field, which is either a fringing field or a parallel transverse field as described herein. This particular device configuration is distinctly different from the thin film version of the modulator/scanner which was described in the copending application Ser. No. D79004, in which light is coupled into a guided mode in a thin film device. The thick film version may involve multiple bounces off one or two TIR surfaces but does not make use of guided modes. The thick film version of the present invention has the potential advantage in that it works at low voltages with high diffraction efficiency, without the problems encountered in coupling light efficiently into a thin film. In addition, the parallel field version of the device has potential for substantially reducing the total length of a device having a fixed number of resolvable elements.